In general, cement has been produced in plants provided with a suspension preheater system or systems having a calcining furnace. The calcining furnace comprises a cylinder and an inverted frustoconical section extending downwardly from the former. The top of the cylinder is connected through an outlet duct with a cyclone separator or separators. Particles to be calcined are charged into a kiln exhaust gas duct and carried by kiln exhaust gases upward into the inverted frustoconical section where they are mixed with combustion air introduced through an air duct. The particles are calcined as they spiral upward in the furnace.
The calcining furnace of the type described is not provided with a throttle chamber and an outlet swirling chamber at the top of the cylinder. Therefore, in order to increase the tarrying time of particles in the calcining furnace, the height of the furnace and/or the inner diameter thereof must be increased. As a result, there arise various problems such as the increase in initial cost.
When the exhaust gases from one calcining furnace are distributed into a plurality of suspension preheater systems, it is preferable to provide a cyclone separator for each preheater system. The exhaust gases having the calcined particles entrained therein are generally swirling when discharged from the furnace so that the calcined particles are, by virtue of their inertia, more concentrated along the outside wall section of the outlet duct which corresponds to the radially outward portion of the vortex than along the wall section corresponding to the radially inward portion of the vortex. As a result, when the exhaust gases are distributed into a plurality of cyclone separators, the calcined particles are varied in concentration from one cyclone separator to another and consequently the operations of the suspension preheater systems including the cyclone separators are not balanced.